Conventional engine valves (intake or exhaust) used to control the flow into and out of the cylinders of internal combustion engines, are controlled by camshafts that fix the opening and closing times of the valves relative to a crankshaft position. While this may be generally adequate, it is not optimal, since the preferred intake and exhaust valve timing varies under varying speeds and loads of the engine. Variable valve timing can account for such conditions as throttling effect at idle, EGR overlap, etc., to substantially improve overall engine performance. Although some attempts have been made to allow for variable timing based upon adjustments in the camshaft rotation, this is still limited by the individual cam lobes themselves.
Consequently, some others have attempted to do away with camshafts altogether by individually actuating the engine valves by some type of electromechanical or electrohydraulic means. These systems have not generally proven successful, however, due to substantial costs, increased noise, reduced reliability, slow response time, and/or increased energy consumption of the systems themselves.
One type of electromechanical system attempted employs simple electromagnets for actuators. But these have proven inadequate because they do not create enough magnetic force for speed needed to operate the valves without an inordinate amount of energy input. This is particularly true in light of the fact that the force profile is not desirable. The magnetic force increases as an armature disk approaches the electromagnet, causing a slap at end of stroke, creating noise and wear concerns, but not much force is available for acceleration at the beginning of the stroke, creating slow response time.
U.S. Pat. No. 5,222,714 attempts to overcome some of the deficiencies of an electromagnetic system by providing a spring to create an oscillating system about a neutral point wherein the springs are the main driving force during operation, and electromagnets provide holding forces in the opened and closed position while also making up for frictional losses of the system.
However, the system in this patent still suffers from some deficiencies not present in prior cam driven systems. For instance, since the electromagnets act on the plate, not the valve head, thermal expansion of the valve stem and manufacturing tolerances can mean that when the plate is in contact with the magnet, the valve may not be fully closed. One way to avoid this problem is for the plate to be designed so that even under the worst condition a gap remains between the magnet and plate, with a large gap at the other extreme of tolerances. To account for this possible large gap then, the current must be increased to hold the plate against the spring with the large gap, increasing energy consumption and heat of the system, and making the actual seating force unknown for any given assembly. Further, to assure closing of the engine valve head with these tolerances, the engine valve can seat with substantial velocity, resulting in unwanted noise and wear.
A consistent, known seating force is desirable for closing the engine valve in its valve seat. Further, it is also desirable for the system to take into account manufacturing tolerances and temperature variations without having to significantly increase the power consumption of the actuator.
Hence, a simple, reliable, fast yet energy efficient actuator for engine valves is also desired, with the flexibility to vary valve timing to substantially improve engine performance.